New Zealand's flag debate has bought forward many different suggestions.

Just imagine what would happen if Labor's Bill Shorten proposed a referendum to change Australia's flag.

He would be carved up by many on the conservative side of politics, denounced for attacking a sacred national symbol.

It is too absurd to even hypothesise about Tony Abbott suggesting a new flag. The Howard government brought in legislation to give maximum protection to the existing one.

Yet Abbott's New Zealand counterpart, Prime Minister John Key, is arguing for the replacement of his country's flag, which is very similar to Australia's.

In a speech on the issue on Tuesday, he promised a referendum on altering the flag if he is re-elected.

New Zealand's flag debate has bought forward many different suggestions.

Key, who has just announced the New Zealand election will be in September, said that once next year's centenary of Gallipoli had passed, "it will be time for us to take some decisions about how we present ourselves to the world beyond 2015".

"The current flag represents the thinking by and about a young country moving from the 1800s to the 1900s," he said.

"It's my belief, and I think one increasingly shared by many New Zealanders, that the design of the New Zealand flag symbolises a colonial and post-colonial era whose time has passed. The flag remains dominated by the Union Jack in a way that we ourselves are no longer dominated by the United Kingdom."

Contrast Abbott's comments on the Australian flag in 2010: "I think it represents our history and I think it represents our future."

Notably, Key's argument for change is accompanied by his support for NZ keeping its tie to the monarchy.

"Our status as a constitutional monarchy continues to serve us well. It's an arrangement that provides stability, continuity and keeps our head of state above party politics."

He rejected the view that the flag should be looked at only if constitutional arrangements were being reviewed.

In Australia, the (at present dormant) republic debate has been seen as the important one, ahead of the flag issue, although if there were an Australian republic, pressure would come on to change the flag, not least because it includes the Union Jack.

One strong criticism levelled against those who have advocated a new flag for Australia is that so many Australians fought and died under the existing one. Those representing returned service men and women in New Zealand make the same case.

But Key said that "being respectful of our history does not lock us permanently in the past" and used the military milepost to support his case.

The Gallipoli centenary would be marked with the present flag, he said, but as New Zealanders reflected on past and future in that context, it was "an appropriate time to write one small but significant new chapter in our national story by reconsidering the flag".

Key is seeking to make the centenary a focal point for new thinking about asserting Kiwi identity.

He proposes creating a cross-party group to oversee the flag consideration process, stressing that "all voices need an opportunity to be heard". He doesn't underestimate the resistance to change, admitting it is "very possible" that the outcome of the process will be that the present flag stays.

Key's preference is for the country's silver fern as a new flag. That is on NZ war graves abroad. But he said he was open to other designs.

"We want a design that says 'New Zealand' in the same way that the maple leaf says 'Canada', or the Union Jack says 'Britain', without a word being spoken, or a bar of those countries' anthems being heard."

That New Zealand is considering a flag change doesn't mean Australia should follow suit. There are persuasive arguments both ways, including the advantage of historical continuity.

But what John Key is doing should be kept in mind next time some Australian political figure suggests anyone who urges a new flag is unpatriotic or worse.

Michelle Grattan is a political fellow at the University of Canberra and chief political correspondent for The Conversation